The UK Prospective Diabetes Study (UKPDS) is a prospective study set up to determine whether improved blood glucose control of non-insulin dependent diabetes will reduce morbidity and mortality. 4,079 patients have been recruited, with mean age 51 years, mean fasting plasma glucose 207mg/dl. It is proposed that the study continues to 1994 with 8 years median follow up. The allocation to "active therapy" with either sulphonylurea or insulin has provided improved blood glucose control compared with allocation to a "diet policy" (median fasting plasma glucose 6.7 and 8.8 mmol/l respectively). Since 1982, patients have had their fundi assessed by quantitation of retinal photographs at diagnosis and at three yearly intervals. At presentation 30% of patients have retinopathy, defined as microaneurysms, and 8% have more severe retinopathy. Since 1987 the retinopathy has been assessed by a modified "191" grading and over three years there is an increase of marked retinopathy, defined as grade 31 or worse, by 2 or more grades in 8% of patients. Already 44 patients have had photocoagulation, 26 patients have had cataract excision, and 20 patients have been registered blind (visual acuity less than 6/60). The main analysis will be to determine whether the policy of improved glucose control will reduce the progress of retinopathy assessed by the incidence of blindness, proliferative retinopathy, maculopathy or by deterioration of the "191" grading. The study will examine both primary prevention and secondary prevention. In addition, several clinical and biochemical variables are being quantitated, eg blood pressure, smoking, plasma lipids, microalbuminuria, and the degree to which these are associated with the progress of retinopathy will also be assessed.